micro rig for 40 meters...

[KA2DZT]

For the modulators, leave the HV on and key the screen voltage.

Fred

[N2DTS]

That's an idea...

I did some tests, I changed the vfo/exciter power supply to a full wave center tap grounded setup (from a bridge) and got 200 and something volts (too low) then went cap input on the power supply and got 400 something volts, but had a hard time getting 20 watts out of the 6146.
I did not try plugging in a 2E26 instead, or changing the pie net coil values.

I think I need to forget about the vfo/exciter as a transmitter and just build something for plate modulation from the ground up.


 

[VE3LYX]

40M isn't bad for low power when things are right. I have been working NJ a bit from here (369miles ATCF) with a single tube AM rig measuring 9.462 watts input at it best.
Also built a 12ax7 speech amp/mod pushing a 6146B (also  osc buffer is a 12ax7) Screen mod/ controlled carrier rig for 20M. Appears to work well but I built it for 20 so it has yet to prove itself. (I am beginning to believe 20M AM is a myth as I have yet to hear anyone)
Anyway I find your project very interesting. When you free build there are always little obstacles but when it all comes together which it eventually does it is well worth the grief. If you want to on air test anytime on 40M just PM me and I will be there to listen.
(cause I know what it is like trying out a QRP style rig .Sort of like a midget trying to pick apples in a crowd. Everyone is taller and you could get trampled never mind the fact you can barely reach) Don

[N2DTS]

Well, I must say, I have no problem making contacts on 80 and 40 meters with my 5 watt Argonaut 5.

The micro rig project is on hold for the moment, with the parts I have, and the chassis, I could not come up with anything that fit on a 12x10x2 inch chassis.
Even a power supply-modulator was a tight fit.

Then I got to figure out where the large micro rig would go in the shack....

[VE3LYX]

Here is my micro rig for 40M. a 56 tube osc driving a 42 tube PA loop modulated in the PA with an absorption loop in south of the antenna link . That loop is connected to a carbon mic giving you" take away AM." (Doesn't add to the carrier, just steals from output in synch with your voice. ) Power is currently around 4 to 5 watts out.
It is also a receiver with the flick of two switches. Voltage drops to jut below oscillation which is what a regen is(an ALMOST oscillating oscillator) and the PA tank is switched out and the Phones switched in. Fun? Well I guess!!!!!
Could be done with no significant changes with a 6J5 and a 6l6.
The OSc regen is just a simple Hartley (cathode centre tap) Regen is controlled with a passive absorption coil and a 25K pot. Voltage is dropped for rx with a 100K resistor.

Needs a RIT for better or easier performance and that will happen. Since circuits are just normal Hartey and normal audio amp a schematic is probably unnecessary. I leave the PA in the A zone  while on Transmit with the RF. Still does the job. Started out as a curiosity and is becoming a lot of fun.
I have outboard power supplies for all my homebuilts so just plug the power cable with its octal male plug into the appropriate  PS when I need to use one of my homebrew rigs. (So two power supplys for 12 rigs)It is about 6 inches high, 7 inches wide and deep and the chassis came from the woods. (Shock safe!)
Don

[N2DTS]

Neat rig!

Well, I was looking for about 20 watts out, and very good clean audio.

Making anything that fits that description seems to get large.

VFO, buffer, driver tube, output tube/tubes, power supply (450 volts at 200? ma), modulator (phaze inverter, output tubes, regulated screen voltage, bias supply).

It does not help that I tend to build everything so its good for twice the rated power and that I like everything to be adjustable.

It seems I might be able to make a nice 20 watt rig in a DX100 size box if I remote the power supply's...

[VE3LYX]

Sub the 6l6 with a 6146B. The 6j5 with a 6sn7. Use the extra triode section to drive the 6146B screen and use the carbon mic circuit with a reversed audio transformer to drive the audio section of the 6sn7. If you drive the screen direct with a .1 cap and drop the screen resting voltage carefully you will get smooth working carrier control (I know  I know but I like it. It gives the power supply a break at rest) and it will stand up smoothly to full power as soon as you speak and if decay isn't too fast (a cap thing) it will not drop out between words.
In fact I have a similar deal in another one.
Don

[VE3AJM]

I've had good luck using my CE 200V on AM on 40m for lower power operation. I was running as low as 5 to 8 watts of carrier and getting 20 over S9 reports in Brampton ON and near Rochester NY. Its a simple to operate no tune tx for my low power AM with good quality audio approaching 100% modulation. There is no instability/FMing/distortion and the station on the other end typically doesn't have to strain/stress out to receive the signal as long as the carrier is of reasonable strength.

This sort of operation on 40m works OK until you have to deal with QRM from slopbuckets who are running power. If you can't put more fire in the wire at that point, you have to fold up and cower away.

Al VE3AJM

[N2DTS]

I tried an experiment with my home brew vfo/exciter.
I made some changes to the plate bypass cap and the screen bypass cap (both very large), then found a nice
8 ohm to 5000 or 20,000 ohm single ended output transformer, only good for 5 watts, but 50 to 20,000 Hz response.
It modulated the rig ok, but during tests, something went pop and I had no audio.
I was dumping 20 watts into it, but it still measures ok on the ohm meter.

I was really pushing that transformer, 750 volts on the plate of the 6146, and 20 watts of audio drive.
I guess I should build a modulator for the rig and use a real mod transformer and see what happens.

[KA2DZT]

Remember, 10Lbs of sardines still will not fit in a 5Lb can, even if you jump on them.

Fred

[N2DTS]

Yes, it was just a test.
I need to figure out/measure the screen current and bias voltage on the 6146 before it will be ready for operation with a modulator.

I also need to figure out the plate load of it, and maybe up the voltage on the buffer/driver tube, or add a driver tube, or change it to two 2E26's and lower the plate voltage.

I have an extra socket, from when I thought I would need a driver tube, but for an exciter, the buffer drives the grid just fine.

[N2DTS]

I changed the power supply to full wave center tapped and did some tests on the VFO/exciter.
I need to check things because with a bridge setup choke input, I get 750 volts out of the supply, and with the center tap setup I get about 240 volts.

That is not right at all.
240 volts, should be 480 volts in bridge mode...

If I cap input the 240 volts, I get about 360.

At 360 volts I was able to get 15 watts out.
450 or 480 volts should make 20 watts easy.

Next to measure screen voltage and current, bias voltage, and try and add neutralization.

[kb3ouk]

I was playing around with the idea of building a low power rig the other day, digging through my box of tubes I found a 6F6G, which the ARRL handbook says is good for 6 watts out with a 275 volt power supply. Might even do like Don and use loop modulation, use something else as an oscillator and use the 6F6 as a power amplifier, then put the absorption loop on the output of the 6F6.

[VE3LYX]

I am waiting. Sounds like a good idea. For the loop modulators net meeting on 7160kc. Currently there seems to be only one member which is really no fun. Another would be welcome. Loop modulating the PA tank is very interesting. You only get modulation if the PA is properly tuned. Makes sense I guess but it surprised me a bit. In a three stage rig you can loop mod the buffer.
I made one for 20M , a solid state rig , MPS102 as a hartley osc driving a generic npn transistor as a buffer amp with a broadbanded coil (very little C mostly L) in the collector circuit. It uses a typical link winding out and south of that has another identical winding on the outboard end of the coil form that is connected to a carbon mic . The link winding feeds a driver transistor which drives a power transistor into a T network tuned by a polycap from an old transistor radio. I have a 80M rockbound QRP solid state rig I tired this on and it works quite well so that inspired this piece. Loop modulating is interesting to say the least. Having said that I also have a 4 tube HB transceiver that has a screen modulated 6l6g in its PA, pictured below. It tunes 80M and 40M.
don

[N2DTS]

Total failure!
After a LOT of work, I added neutralization.
No space on the micro chassis so I had to move things around somewhat.

I never had such a mess of a tune up. It seems to be super un neutralized now, it acts totally crazy.
Back to the drawing board.

[VE3LYX]

Be not discouraged! Out of failure and determination comes success and knowledge, always, without exception. You have an idea. It is probably a good one. See it thru
don

[N2DTS]

Yes, I need to change values.
The cap to ground that sets the neutralization is way too small I think, allowing way too much feedback.

I will use a shorter pickup stub and a put a fixed cap across the adjustable cap and see what happens.

[N2DTS]

I played around with values in the neutralization circuit and did not get anywhere, so I then did some measurements and tests.
The screen voltage was very low at 75 volts, so I bypassed a resistor and got it adjustable up to 150 volts.
I then played around with the tank coil, I had too many turns, and removing some got me up to 20 watts out.
Peak power is not the same as cathode current dip, but at dip I get 20 watts out.

Maybe its time to think about building a modulator.

[KA2DZT]

I lost track on what we're doing with this project.  What tube is the PA?  Was it a 6146 or 2E26?  I think I fell asleep through some of the posts, so I need to catch up.  I like to stay on top of the ongoing projects just in case I'm needed to resolve any problems.  Provided I don't fall asleep again.

Fred

[N2DTS]

Well, as it stands, I will try and modulate the vfo-exciter I built a few months ago.
Its got a 6146 in the output.
Picture:

http://n2dts.smugmug.com/Ham-radio/i-ZKsgzjr

VFO (on 80 meters), buffer tube, 6146 with the grid tuned to 40 meters, so I could run the vfo all the time to work with the frequency counter, and for stablility.

The plan is to build a 6l6 modulator and power supply, and use a 25 watt mod transformer I have on hand.

The vfo/exciter has a separate power supply, and I am not sure if I want to do the same for the modulator.

Power supply pic:

http://n2dts.smugmug.com/Ham-radio/i-vvdDhDJ